1. Field of the Invention
This invention relates to the field of toughened polyamides, and more particularly to the field of polyamides that are toughened by the addition of tougheners such as ionomers and ethylene copolymers.
2. Description of the Related Art
Improvement of impact strength, or toughness, of polyamide resins has long been of interest. Resistance to shattering or brittle breaking on impact of polyamide molded articles is a desirable feature of any molded article. Any tendency to break on impact in a brittle fashion (rather than ductile fashion) is a significant limitation on the usefulness of such articles. By xe2x80x9cductilexe2x80x9d is meant that sharp cracks in the molded resin are not initiated, or if initiated do not tend to propagate, from the area of impact. Breaks in ductile materials are characterized more by tearing with a large volume of adjacent material yielding at the edge of the crack or tearing rather than a sharp, clean break with little molecular displacement. A resin having good ductility is one that is resistant to crack propagation caused by impact.
A variety of additives have been added to polyamide resins to improve strength and ductility. For example, U.S. Pat. No. 4,174,358 issued Nov. 13, 1979 to Epstein, describes improving impact strength and ductility by adding a selected random copolymer which adheres to the polyamide. U.S. Pat. No. 4,594,386 issued Jun. 10, 1986 to Olivier, describes improving impact strength and toughness of polyamide resins by blending polyamide resin with maleic anhydride grafted EPM rubber of low molecular weight. U.S. Pat. No. 4,346,194 issued Aug. 24, 1982 to Roura, describes a polyamide molding material having good impact strength at low temperatures, e.g., 0xc2x0 C.; the polyamide molding material comprises a nylon 6/66 blend and a toughening copolymer that is an adduct of a polymer of ethylene, at least one C3-C6 alpha-olefin and at least one non-conjugated diene, with an unsaturated compound containing carboxyl or carboxyl derivatives.
The inverse relationship between percent neutralization and ionic polymer flow rate, such as might be measured by melt index, i.e., M.I., is well known. This inverse relationship can present a processing problem in cases where it may be desired to process a highly neutralized ionomer resin that contains even moderate levels of methacrylic acid. While it can be inferred that a higher degree of neutralization of acid groups in the resin can lead to greater toughening, ultimate flow rates at the highest levels of neutralization, as measured by M.I., approach a xe2x80x9cno flowxe2x80x9d condition, i.e., the resin blend becomes intractable. In U.S. Pat. No. 5,688,868 issued Nov. 18, 1997 to Fish, there is disclosed a process for preparing blends of polyamide and ionic copolymers which have very high degrees of neutralization by adding metal ions to the blend in situ in amount which exceed that needed to neutralize 100% of the acid molecules present-in the blend without incurring a xe2x80x9cno flowxe2x80x9d condition.
In addition, U.S. Pat. Nos. 4,346,194 and 4,478,978 to Roura disclose toughened blends of nylon 6,6 and nylon 6.
Although the previously known combinations impart improved toughness to polyamide resins, it has now been found that certain novel compositions provide further improvements in polyamide resin toughness at very low temperatures, including temperatures less than about xe2x88x9230xc2x0 C. This invention is directed to a polyamide molding blend suitable for making molded articles for use in applications requiring toughness at very low temperatures.
The present invention is directed to the discovery that the toughness of a polyamide that includes an EP or EPDM grafted with 0.05 to 3 weight percent of a carboxylic acid or an anhydride thereof can be unexpectedly increased by the addition of a particular ionomer at least partially dispersed in nylon 6, provided that the number of moles of selected metal ions in the ionomer is greater than or equal to the number of moles of the carboxylic acid in the grafted EP or EPDM.
The present invention relates to a multiphase polymer composition having increased impact resistance at low temperatures, that includes a blend of
(1) an ionomer at least partially dispersed in nylon 6, the ionomer being formed from a partially neutralized ethylene acid interpolymer precursor, the ethylene acid interpolymer precursor having polymerized in-chain units derived from the monomers comprising:
(a) ethylene,
(b) 2 to 25 weight percent of an acid selected from the group of acrylic acid, methacrylic acid, and mixtures thereof,
(c) 0.1 to 15 weight percent of a dicarboxylic acid monomer selected from the group consisting of maleic acid, fumaric acid, itaconic acid, maleic anhydride, itaconic anhydride, a C1-C4-alkyl half ester of maleic acid, and a mixture of these dicarboxylic acid monomers,
(d) 0-40 weight percent of a C1-C8-alkyl alkyl acrylate,
the ionomer being formed by neutralization of from about 5 to 90 percent of the total number of carboxylic acid units in the copolymer with metal ions selected from the group of zinc, magnesium, manganese and mixtures thereof, alone or in combination with sodium or lithium ions,
with the proviso that the total of acrylic acid, methacrylic acid, and dicarboxylic acid monomer is from 4 to 26 weight percent of the acid copolymer precursor,
the ionomer having a melt index of from 0.01 to 100 grams/10 minutes; and
(2) a polyamide other than nylon 6, and
(3) a composition that includes an elastomer selected from the group of EP, EPDM and styrenic thermoplastic elastomer, which elastomer is grafted with 0.05 to 3 weight percent of a carboxylic acid or any anhydride thereof;
wherein the ratio of the number of moles of metal ions in the ionomer to the number of moles of the carboxylic acid or any anhydride thereof in the grafted EP, EPDM or styrenic thermoplastic elastomer is greater than or equal to 1.0.